Organometallic, salts prepared from fatty acids are frequently incorporated into oils and greases to provide lubricating compositions having special properties (see e.g. Synthetic Lubricants And High-Performance Functional Fluids, Edited by Leslie R. Rudnick and Ronald L. Shubkin, CRC Press 1999). In particular, saturated and unsaturated carboxylic acid salts are well known friction-reducing additives in lubricating oils, (Spikes, H. A. “Boundary Lubrication and Boundary Films.” Proc. 19th Leeds-Lyon Symposium on Tribology, Leeds, September 1992; Thin Films in Tribology, ed. D Dowson et al., Elsevier 1993). The organometallic salts can be based on different metal elements as noted in the Fuels and Lubricants Handbook: Technology Properties Performance and Testing Edited by George E Totten, Steven R. Vestbrook, Rajesh Shah (2003). Copper based additives are often preferred because of their effectiveness in lubricants. Several types of copper compounds including copper dithiophosphates, dithiocarbamates, sulphonates, carboxylates, acetylacetones, phenates, copper stearate and palmitate have showed significantly lower friction and wear. Copper carboxylates, for example copper oleate, have also been used as an antioxidant, (British Pat. No. 2,056,482 and in European Pat. No. 92946 as an engine oil antioxidant). Copper-based organometallic compounds can give maximum benefit when used as multifunctional additives to reduce friction and wear in liquid lubricants or greases, fuels, cutting fluids, and hydraulic fluids. Despite all the advances in copper based lubricant additives and lubricant oil formulation technology, there remains a need for lubricant oil additives that provide even more superior wear protection and environmentally beneficial properties such as reduced exhaust emissions.
Organometallic salts useful as lubricant additives can be synthesised using a number of different reaction routes. Metal carboxylates, in particular metal salts based on long chain unsaturated or saturated fatty acids, are commonly prepared by reacting a metal carbonate with a fatty acid. One well-known method to make copper oleate is heating oleic acid with copper carbonate, (U.S. Pat. No. 1,013,538). Another process is by mixing equimolar aqueous solutions of sodium oleate and inorganic soluble salts of the desired metal, for example copper chloride. The resultant metal oleate will precipitate and it is then filtered, washed and dried; (Ratoi, M., Bovington, C. and Spikes, H. (2000) Mechanism of metal carboxylate friction modifier additive behaviour; International Tribology Conference, Nagasaki, JP).
The design and development of a lubricant additive to provide and impart the desired properties when added to a lubricant formulation is an unpredictable and challenging process. Moreover, the physical properties, solubility and performance of a metal carboxylate additive cannot be anticipated or determined by the chemical structure of such an organometallic compound alone. These factors do not follow simple structure-activity relationships, (Kenbeek, D., Buenemann, T., and Rieffe, H., Review of Organic Friction Modifiers—Contribution to Fuel Efficiency, SAE Technical Paper 2000-01-1792, 2000).
Most lubricant compositions include a base oil. Generally this base oil is a hydrocarbon oil or a combination of hydrocarbon oils. The hydrocarbon oils have been designated by the American Petroleum Institute as falling into Group I, II, III or IV. Of these, the Group I, II, and III oils are natural mineral oils. Group I oils are composed of fractionally distilled petroleum which is further refined with solvent extraction processes to improve properties such as oxidation resistance, and to remove wax. Group II oils are composed of fractionally distilled petroleum that has been hydrocracked to further refine and purify it. Group III oils have similar characteristics to Group II oils, with Groups II and III both being highly hydro-processed oils which have undergone various steps to improve their physical properties. Group III oils have higher viscosity indexes than Group II oils, and are prepared by either further hydrocracking of Group II oils, or by hydrocracking of hydro-isomerized slack wax, which is a byproduct of the dewaxing process used for many of the oils in general. Group IV oils are synthetic hydrocarbon oils, which are also referred to as polyalphaolefins (PAOs).
In order to modify the lubrication properties of the various base oils, additives are frequently employed. These additives include materials designed to function, for example, as antiwear agents, friction reducing additives, antioxidants, dispersants, detergents, extreme pressure additives, and corrosion inhibitors. It is highly desirable that all additives are soluble in a wide range of base oils. Good additive solubility is important to ensure that the formulated lubricant is stable with no tendency to separate or form sediments. It is also important to ensure that the additives are properly solubilized in order to enable them to function properly and perform effectively. Additive solubility is desirably maintained across a wide range of temperature and other conditions, in order to enable shipping, storage, and/or relatively prolonged use of these compositions. However, attainment of these desirable qualities should not be at the expense of overall performance. Unfortunately, some additives that provide as at least one benefit, for example friction reduction or protection against wear, also suffer from low solubility and are, therefore, of limited commercial value.
Those skilled in the art have attempted to develop alternative solutions to try and deploy additives with low solubility in lubricant formulations. One approach has been to include one or more co-base oils, such as synthetic esters or vegetable oils, in the lubricant composition. For example, esters have been used as co-base oils with polyalphaolefins for this purpose. Unfortunately, such esters often suffer from poor hydrolytic stability and thus may represent an unacceptable sacrifice in overall performance in order to achieve a remedy for the solubility problem.
Another approach to solve the problem of low solubility has been to use alternative lubricant additives containing high levels of zinc, sulphur, and/or phosphorus. These lubricant additives can offer adequate performance in terms of friction reduction and wear protection. They are, however, often less effective compared to the superior and more desirable additives based on low phosphorous, low sulphur and low sulphated ash technology.
The prior art also shows that there is a group of non-soluble lubricant additives that, depending on their structure, reduce friction and provide wear protection in a mechanical fashion by preventing direct contact between metal surfaces. Examples of additives that function in this manner are molybdenum disulphide and Teflon® fluorocarbon polymer (PTFE). These additives can be used successfully in grease compositions; however, they are not effective in lubricant oil compositions. The lubricants have been found to suffer from poor stability due to agglomeration and sedimentation of insoluble materials. As a consequence, the performance deteriorates over time and becomes unacceptable, especially in terms of friction and wear.
Yet another group of additives with low solubility consists of metal powders, for example copper alloys. These are claimed to reduce friction and wear. They are capable of forming a metal layer on the friction surfaces when deployed in lubricants. The tribo-layer is deposited on the metal surface due to physical and chemical processes. It improves the frictional conditions on the metal surfaces of moving parts and increases the loading resistance of the surfaces. These lubricant compositions, however, have been found to suffer from poor stability due to agglomeration and sedimentation of insoluble materials. As a consequence, also their performance deteriorates over time and becomes unacceptable, especially in terms of friction and wear.
A preferred group of lubricant additives that is useful in order to reduce friction and wear is that based on organometallic salts. Examples are described in Lubricant Additives: Chemistry and Applications, Second Edition, edited by Leslie R. Rudnick, CRC Press, 2009 which document is included by reference for the purpose of disclosure. It includes for example a number of copper and molybdenum compounds; specific examples are copper oleate, copper salicylate, copper naphthenate, and molybdenum naphthenate. These additives can function as very effective friction reducers and antiwear agents when used individually or preferably in combination with other compounds. The disadvantage of this group of materials is that they are most often solids at ambient temperatures and have limited oil solubility, especially when used in more saturated and paraffinic hydrocracked or synthetic base oils like Group II, III and Group IV (PAO). This limits the use of these additives in high performance automotive, industrial and off-highway lubricants.
Although the above prior art shows that useful additive compositions are available, it also demonstrates that there are significant shortcomings. There continues to be a need for high performance lubricant additive compositions that are soluble, especially when used in more saturated and paraffinic hydrocracked or synthetic base oils like Group II, III and Group IV (PAO). These important improvements are achieved in the present invention.
It has been found that copper oleate, which has a melting point of about 55° C., is significantly soluble in Group I base oils but it only has limited solubility in Group II, III, and IV base oils. This prevents copper oleate being deployed on its own or in combination with other suitable components to formulate lubricants for many applications that require Group II, III and IV higher quality base oils.
In U.S. Pat. No. 5,994,277 is disclosed a composition for improving the antioxidancy of crankcase lubricants. The composition includes three essential components, namely copper, molybdenum and one or more oil soluble aromatic amines. The copper may be added in the form of a salt of a C8 to C15 fatty acid. The molybdenum is preferably added in the form of an oil-soluble molybdenum carboxylate. The aromatic amine or mixture of aromatic amines may be an alkylated diphenylamine. An example is given where the copper is added as copper oleate and the molybdenum as molybdenum 2-ethylhexanoate.